Micro-LED module and method for fabricating the same

ABSTRACT

A method for fabricating a micro-LED module is disclosed. The method includes: preparing a micro-LED including a plurality of electrode pads and a plurality of LED cells; preparing a submount substrate including a plurality of electrodes corresponding to the plurality of electrode pads; and flip-bonding the micro-LED to the submount substrate through a plurality of solders located between the plurality of electrode pads and the plurality of electrodes. The flip-bonding includes heating the plurality of solders by a laser.

This is a continuation of U.S. application Ser. No. 15/818,738, filedNov. 20, 2017, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method for fabricating a micro-LEDmodule by flip-bonding a micro-LED including a light-transmittingsubstrate to a submount substrate having a coefficient of thermalexpansion significantly different from the micro-LED.

2. Description of the Related Art

Displays using micro-LED modules are known. Each of the micro-LEDmodules is fabricated by flip-bonding a micro-LED including a pluralityof LED cells to a submount substrate.

Generally, the micro-LED includes a light-transmitting sapphiresubstrate and a gallium nitride semiconductor light emitting unit formedon the light-transmitting sapphire substrate and having a plurality ofLED cells. The semiconductor light emitting unit includes an exposedarea of an n-type semiconductor layer formed by etching and theplurality of LED cells are arranged in a matrix on the exposed area ofthe n-type semiconductor layer. Each of the LED cells includes an n-typesemiconductor layer, an active layer, and a p-type conductivesemiconductor layer. A p-type electrode pad is disposed on the p-typeconductive semiconductor layer of each LED cell. An n-type electrode padis disposed on the exposed area of the n-type semiconductor layer.

The submount substrate includes a plurality of electrodes disposedcorresponding to the electrode pads of the micro-LED. The micro-LED isflip-bonded to the mount substrate through solder bumps so that theelectrode pads of the micro-LED are connected to the electrodes of thesubmount substrate. For flip-bonding of the micro-LED to the submountsubstrate, a solder constituting at least a portion of each solder bumpshould be heated to a temperature around its melting point. However,there are large differences in expansion and contraction strains betweenthe Si-based submount substrate and the sapphire substrate upon heatingand cooling during flip-bonding because the coefficient of thermalexpansion of the Si-based submount substrate is significantly differentfrom that of the sapphire substrate. These differences cause a seriousmisalignment between the submount substrate and the micro-LED. Due tothis misalignment, the electrode pads of the micro-LED are not connectedto the electrodes of the submount substrate, and in a more severe case,the electrode pads of the micro-LED are misconnected to the electrodesof the submount substrate, causing serious detects, such as electricalshorting.

For example, the sapphire substrate on which the micro-LED is based hasa coefficient of thermal expansion of 7.6 μmm⁻¹K, and the Si-basedsubmount substrate has a coefficient of thermal expansion of 2.6 μmm⁻¹K.That is, the coefficient of thermal expansion of the sapphire substrateamounts to about 2.5 times that of the Si-based submount substrate. Thecoefficients of thermal expansion of the substrates depend on theirtemperature. The use of a high melting point solder for the bumpsrequires a high flip-bonding temperature. In this case, the largedifference in coefficient of thermal expansion between the sapphiresubstrate and the submount substrate causes a misalignment between themicro-LED and the submount substrate, making it difficult to bond themicro-LED to the submount substrate. For example, when the melting point(260° C.) of the solder is set as a bonding temperature, an misalignmentof 5 to 6 μm is caused per 1 cm of the substrates, making itsubstantially impossible to use the solder in a process where a bondingprecision of 2 μm is required, like flip-bonding of the micro-LED.

Thus, there exists a need in the art for a solution to the problem ofmisalignment caused by a difference in coefficient of thermal expansionbetween a sapphire substrate of a micro-LED and a submount substratewhen the micro-LED is flip-bonded to the submount substrate.

PRIOR ART DOCUMENTS Patent Documents

-   -   (Patent Document 1) Korean Patent No. 10-1150861 (issued on May        22, 2012)    -   (Patent Document 2) Korean Patent No. 10-0470904 (issued on Jan.        31, 2005)

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems and itis an object of the present invention to provide a method forfabricating a micro-LED module that is free from problems associatedwith misalignment caused by a difference in coefficient of thermalexpansion between substrates.

A method for fabricating a micro-LED module according to one aspect ofthe present invention includes: preparing a micro-LED including aplurality of electrode pads and a plurality of LED cells; preparing asubmount substrate including a plurality of electrodes corresponding tothe plurality of electrode pads; and flip-bonding the micro-LED to thesubmount substrate through a plurality of solders located between theplurality of electrode pads and the plurality of electrodes, wherein theflip-bonding includes heating the plurality of solders by a laser.

According to one embodiment, the flip-bonding includes locally heatingthe plurality of solders by a plurality of laser beams.

According to one embodiment, the individual electrode pads are formed onthe LED cells and the flip-bonding includes heating the solders locatedbetween the individual electrode pads and the submount substrate by alaser sequentially passing through the LED cells and the individualelectrode pads.

According to one embodiment, the individual electrode pads aretransmissive to the laser.

According to one embodiment, the individual electrode pads includecavities through which the laser passes.

According to one embodiment, the micro-LED includes a common electrodepad formed on the surface of an epilayer around the plurality of LEDcells and the flip-bonding includes heating a solder located between thecommon electrode pad and the submount substrate by the lasersequentially passing through the epilayer and the common electrode pad.

According to one embodiment, the common electrode pad is transmissive tothe laser.

According to one embodiment, the common electrode pad includes a cavitythrough which the laser passes.

According to one embodiment, the flip-bonding includes heating theplurality of solders by a plurality of laser beams passing verticallythrough the micro-LED from one side to the other side of the micro-LEDand the plurality of laser beams include laser beams passing through thesubstrate and the epilayer in which none of the LED cells are presentand laser beams passing through the substrate and the epilayer in whichthe LED cells are present.

According to one embodiment, the flip-bonding includes heating theplurality of solders by a plurality of laser beams passing verticallythrough the micro-LED from one side to the other side of the micro-LEDand focusing lenses are used to focus the laser beams on thecorresponding solders.

According to one embodiment, the flip-bonding includes placing aplurality of laser beam irradiation units in an arrangementcorresponding to an arrangement of the plurality of solders at one sideof the micro-LED before heating the plurality of solders by a pluralityof laser beams passing vertically through the micro-LED from one side tothe other side of the micro-LED.

According to one embodiment, a plurality of laser beam irradiation unitsare placed in an arrangement corresponding to an arrangement of theplurality of solders at one side of the micro-LED before heating theplurality of solders by a plurality of laser beams passing verticallythrough the micro-LED from one side to the other side of the micro-LEDand each of the plurality of laser beam irradiation units includes anoptical guide connected to a laser source, a collimator for making laserbeams entering through the optical guide parallel to each other, a beamcontroller for controlling the cross-sectional size of the parallellaser beams, and a focusing lens for focusing the parallel laser beamswhose cross-sectional size is controlled on the corresponding solders.

According to one embodiment, the flip-bonding may include matching aplurality of laser beam irradiation units to the plurality of solders ina 1:1 ratio such that the plurality of solders are heated by laser beamsirradiated from the plurality of laser beam irradiation units.

According to one embodiment, the flip-bonding may include matching aplurality of laser beam irradiation units to the plurality of solders ina 1:n (where n is a natural number equal to or greater than 2) ratiosuch that two or more of the solders are heated by laser beamsirradiated from each of the laser beam irradiation units moving in alinear or zigzag pattern.

According to one embodiment, the flip-bonding may include matching twoor more laser beam irradiation units to two or more solder groups suchthat each of the laser beam irradiation units heats the solders in thecorresponding solder group.

A micro-LED module according to a further aspect of the presentinvention includes: a micro-LED including a substrate, an epilayerincluding a plurality of LED cells, second conductive individualelectrode pads disposed on the plurality of LED cells, and a firstconductive common electrode disposed around the plurality of LED cells;a submount substrate including a plurality of electrodes correspondingto the individual electrode pads and the common electrode pad; andsolders located between the electrodes and the individual and commonelectrode pads, wherein the solders are heated by a laser and are thenhardened so that the electrodes are connected to the individualelectrode pads and the common electrode pad.

According to one embodiment, the substrate, the epilayer, the individualelectrode pads, and the common electrode pad are transmissive to laserbeams such that the solders are heated by the laser passing through themicro-LED from one side to the other side of the micro-LED.

According to one embodiment, the individual electrode pads may be madeof a laser beam-transmitting material.

According to one embodiment, the individual electrode pads may includecavities through which the laser passes.

According to one embodiment, the common electrode pad may be made of alaser beam-transmitting material.

According to one embodiment, the common electrode pad may include acavity through which the laser passes.

The micro-LED module of the present invention is constructed such thatlaser beams are locally irradiated onto the solders located between themicro-LED and the submount substrate to rapidly melt the solders. Due tothis construction, no substantial heat is applied to the laserbeam-transmitting micro-LED and the submount substrate located beyondthe reach of laser beams. Therefore, the present invention can provide asolution to the problem of misalignment caused by a difference incoefficient of thermal expansion between the micro-LED and the submountsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIGS. 1a to 1e illustrate a process for constructing a micro-LED inaccordance with a first embodiment of the present invention;

FIG. 2 is a partial cross-sectional view illustrating a submountsubstrate used in a first embodiment of the present invention;

FIGS. 3 and 4 illustrate a process for forming bumps including solderson a submount substrate in accordance with a first embodiment of thepresent invention;

FIGS. 5a, 5b, and 5c illustrate a process for flip-bonding a micro-LEDto a submount substrate;

FIG. 6 illustrates an alternative example of a first embodiment of thepresent invention;

FIGS. 7 and 8 illustrate other examples of a first embodiment of thepresent invention;

FIG. 9 illustrates a process for flip-bonding a micro-LED to a submountsubstrate in accordance with a second embodiment of the presentinvention; and

FIG. 10 shows heating-cooling curves of the micro-LED and the submountsubstrate during the flip-bonding process illustrated in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

First and second embodiments of the present invention will now bedescribed with reference to the accompanying drawings. The drawings andthe examples are simplified and exemplified such that those skilled inthe art can readily understand the present invention, and therefore,they should not be construed as limiting the scope of the presentinvention.

First and second embodiments of the present invention provide a methodfor fabricating a micro-LED module by flip-bonding a micro-LED to asubmount substrate as an active matrix substrate. According to the firstand second embodiments of the present invention, first, a Si-basedsubmount substrate including an electric circuit and electrodes isprepared and a micro-LED based on a sapphire substrate is constructed.

The construction of a micro-LED, the formation of bumps, andflip-bonding of the micro-LED to a submount substrate in accordance withthe first and second embodiments of the present invention will beexplained sequentially below.

First Embodiment

Construction of Micro-LED

With reference to FIGS. 1a to 1e , an explanation will be givenregarding a method for constructing a micro-LED.

First, an epilayer including an n-type semiconductor layer 132, anactive layer 133, and a p-type semiconductor layer 134 is formed on theprimary surface of a light-transmitting sapphire substrate 131, asillustrated in FIG. 1a . The light-transmitting sapphire substrate 131has a coefficient of thermal expansion of 7.6 μmm⁻¹K.

Next, the epilayer is etched to a predetermined depth using a maskpattern to form trenches 101 and an exposed area 102 of the n-typesemiconductor layer 132, as illustrated in FIG. 1b . As a result of thisetching, a plurality of LED cells 130 are formed. The plurality of LEDcells 130 are separated by the trenches 101 and are surrounded by theexposed area 102 of the n-type semiconductor layer 132. Each of the LEDcells 130 has a structure in which the active layer 133 and the p-typesemiconductor layer 134 are formed on the n-type semiconductor layer132. Although not illustrated, a buffer layer may be formed between then-type semiconductor layer 132 and the sapphire substrate 131. Othersemiconductor layers having specific functions may be interposed betweenthe n-type semiconductor layer 132 and the active layer 133, between theactive layer 133 and the p-type semiconductor layer 134, and on theexposed surface of the p-type semiconductor layer 134. Since theepilayer and the light-transmitting sapphire substrate 131 aretransmissive to laser beams, the LED cells 130 are also transmissive tolaser beams.

Next, a laser-transmitting p-type electrode pad 150 is formed on each ofthe p-type semiconductor layers 134 of the LED cells 130 and alaser-transmitting n-type electrode pad 140 is formed at the peripheryof the exposed area 102 of the n-type semiconductor layer 132, asillustrated in FIG. 1c . The p-type electrode pad 150 and the n-typeelectrode pad 140 are designed to have different thicknesses. Thisdesign compensates for the step height between the p-type semiconductorlayer 134 and the n-type semiconductor layer 132 and allows the surfaceof the p-type electrode pad 150 to which a solder is to be bonded to lieat the same level as the surface of the n-type electrode pad 140 towhich a solder is to be bonded.

Next, a passivation layer 160 is formed so as to cover the LED cells 130and the exposed area 102 of the n-type semiconductor layer 132, asillustrated in FIG. 1 d.

Next, first holes 162 through which the p-type electrode pads 150 areexposed and a second hole 164 through which the n-type electrode pad 140is exposed are formed, as illustrated in FIG. 1e . The first holes 162and the second hole 164 may be formed by etching using a mask pattern.In this embodiment, the passivation layer 160 is formed to substantiallythe same thickness along the cross-sectional profile of the LED cells130 such that the width and depth of the trenches 101 between theneighboring LED cells 130 decrease but the trenches remain unremoved.Alternatively, the passivation layer 160 may completely fill thetrenches 101.

Preparation of Submount Substrate and Formation of Bumps

Referring first to FIG. 2, a Si-based submount substrate 200 having asize of about 15,000 μm×10,000 μm is prepared, followed by the formationof pillar bumps. The submount substrate 200 may include a plurality ofCMOS cells corresponding to the plurality of LED cells, a plurality ofindividual electrodes 240 corresponding to the p-type electrode pads ofthe micro-LED, and a common electrode (not illustrated) corresponding tothe n-type electrode pad of the micro-LED. The plurality of electrodes240 of the submount substrate 200 are arranged in a matrix on a Si-basedsubstrate material 201 and are connected to the CMOS cells. Apassivation layer 250 is formed so as to cover the electrodes 240. Thepassivation layer 250 has holes 252 through which the individualelectrodes 240 are exposed.

Referring to FIGS. 3 and 4, bumps are formed by a process including thefollowing steps: first scrubbing S101, formation of an under bumpmetallurgy (UBM) S102, photolithography S103, scum removal S104, Cuplating S105, solder metal plating S106, PR stripping S107, UBM etchingS108, second scrubbing S109, reflow S110, and third scrubbing S111.

In S101, a submount substrate 200 is scrubbed with a scrubber, asillustrated in (a) of FIG. 4. In the submount substrate 200, a pad-typeelectrode 240 made of an Al or Cu material is formed on a substratematerial 201 including CMOS cells and a passivation layer 250 having ahole 252 is formed on the substrate material 201. The CMOS cells areformed by a CMOS process and one area of the electrode 240 is exposedthrough the hole 252.

In S102, a UBM 261 is formed on the submount substrate 200 to cover thepassivation layer 250 and the electrode 240, as illustrated in (b) ofFIG. 4. The UBM 261 serves to increase the adhesion of the electrode 240to a Cu pillar and prevent a solder from diffusing. In this embodiment,the UBM 261 may have a layered structure of Ti/Cu and may be formed bysputtering the corresponding metals.

In S103, a photoresist (PR) 300 is formed over the entire area of theUBM 261 on the submount substrate 200, as illustrated in (c) of FIG. 4.Thereafter, a mask pattern is placed (not illustrated) on thephotoresist and light is applied to form a hole 302 through which onlyone area of the UBM 261 formed directly on the electrode 240 is exposed.Next, S104 is carried out to remove scum formed during thephotolithography.

Next, Cu is plated through the opening 302 of the PR 300 to form a Cupillar 262 (S105) and then SnAg as a solder metal is plated on the Cupillar 262 to form a SnAg solder cap 263 in the form of a layer with apredetermined thickness (S106), as illustrated in (d) of FIG. 4. It isnoted herein that Cu may be Cu metal or its alloy.

Next, S107 is carried out to strip the PR. As a result, the upper andside surfaces of a bump including the Cu pillar 262 and the solder 263are exposed, as illustrated in (e) of FIG. 4.

Next, UBM etching is performed such that only the portion of the UBM 261located directly under the Cu pillar 262 remains unremoved and the otherportions of the UBM 261 are removed by etching (S108), as illustrated in(f) of FIG. 4. Then, S109 is carried out to remove residue. After theUBM etching (S109), the resulting bump 260 has a structure in which theCu pillar 262 and the solder cap 263 are sequentially stacked on the UBM261 formed on the electrode 240 of the submount substrate 200. Next,reflow is performed (S110). As a result, the solder 263 in the form of alayer is melted and coagulated to form a hemisphere. Alternatively, thesolder 263 may have a shape whose cross-section is semicircular. Rapidthermal processing (RTP) is suitable for this reflow. Next, thirdscrubbing is performed to remove residue (S111).

Preferably, the interval between the adjacent Cu pillar bumps 260 on thesubmount substrate 200 corresponds to the diameter of the Cu pillar 262.It is desirable that the interval between the adjacent Cu pillar bumps260 does not exceed 5 μm. If the interval exceeds 5 μm, the diameter ofthe Cu pillar bumps 260 and the size of the resulting LED cellsincrease, resulting in a deterioration in the precision of a displayincluding the micro-LED.

Flip-Bonding

Referring to FIGS. 5a, 5b, and 5c , the micro-LED 100 based on thesapphire substrate 131 is flip-bonded to the submount substrate 200based on the Si substrate material. The Si substrate material has acoefficient of thermal expansion of 2.6 μmm⁻¹ K and the sapphiresubstrate 131 has a coefficient of thermal expansion of 7.6 μmm⁻¹K,which is about 2.5-fold higher than that of the Si substrate material.

As mentioned earlier, the plurality of electrodes of the submountsubstrate 200 are disposed corresponding to the electrode pads 150 ofthe micro-LED 100. The bumps 260 are formed on the plurality ofelectrodes. Each of the bumps 260 consists of the Cu pillar 262 and theSnAg solder (i.e. solder cap 263). As mentioned briefly above, the LEDcells 130 and the electrode pads 140 and 150 of the micro-LED 100 aretransmissive to laser beams such that laser beams reach and locally heatthe solders 263. For example, the electrode pads 140 and 150 are made ofa conductive transparent metal compound through which laser beams can betransmitted.

After the solders 263 formed on the pillars 262 are located between theelectrode pads 140 and 150 of the micro-LED 100 and the electrodes ofthe submount substrate 200, local heating of the solders 263 by laserbeams allows the electrode pads 140 and 150 of the micro-LED 100 to bebonded to the electrodes of the submount substrate 200. This bondingwill be explained in detail below.

The micro-LED 100 is flip-bonded to the submount substrate 200 usinglaser beams. To this end, first, it is necessary to arrange theindividual electrode pads 150 disposed on the LED cells 130 of themicro-LED 100 and the common electrode pad 140 disposed at the peripheryof the micro-LED 100 to face the electrodes of the submount substrate200 and locate the solders 263 or the bumps 260 including the solders263 between the electrodes of the submount substrate 200 and theelectrode pads of the micro-LED 100. Thus, the plurality of solders 263are located in an arrangement corresponding to that of the plurality ofelectrode pads 140 and 150 between the micro-LED 100 and the submountsubstrate 200.

Next, a plurality of laser beam irradiation units 1000 are placed in thesame arrangement as that of the solders 263 at the upper side of themicro-LED 100. Each of the laser beam irradiation units 1000 includes anoptical guide 1100 connected to a laser source, a collimator 1200 formaking laser beams entering through the optical guide parallel to eachother, a beam controller 1300 for controlling the cross-sectional sizeof the parallel laser beams, and a focusing lens 1400 for focusing theparallel laser beams whose cross-sectional size is controlled on onepoint. Although not illustrated, each of the laser beam irradiationunits 1000 may further include a laser amplifier, an optical coupler,and a laser oscillation controller. The power of the laser beams isappropriately selected depending on the melting point of the solderingmaterial.

The plurality of laser beam irradiation units 1000 may be operatedsimultaneously. In this case, laser beams L are supplied to thecollimators 1200 through the optical guides 1100, the collimators 1200make the laser beams parallel to each other and output the parallellaser beams, the beam controllers 1300 extend the diameter of theparallel laser beams, and the focusing lenses 1400 allow the parallellaser beams whose diameter is extended to pass through the micro-LED 100and to be focused on the solders 263 in contact with the correspondingelectrode pads 140 and 150 of the micro-LED 100. As a consequence, thesolders 263 focused by the laser beams L are heated and melted. Sincethe laser beams pass through the micro-LED 100 without being focused onthe micro-LED 100, the effect of the laser beams L to heat the micro-LED100 is negligible. Thus, no thermal expansion and contraction occurs inthe micro-LED 100. After being rapidly heated by the laser beams L, thesolders 263 are cooled and hardened to bond the electrode pads 140 and150 of the micro-LED 100 to the electrodes of the submount substrate200. It is preferred that the focusing position of the laser beams L isdetermined between one and two-third of the height of the correspondingsolder before melting. If the focusing position of the laser beam Lexceeds two-third of the height of the solder (that is, it is close tothe micro-LED 100), the LED cell 130 and the electrode pad 140 or 150may be thermally damaged. Meanwhile, if the focusing position of thelaser beam L is less than one-third of the height of the solder, thereis a high risk that a circuit of the submount substrate 200 may bethermally damaged.

ALTERNATIVE EXAMPLE

Alternatively, the electrode pads 140 and 150 may be made of a materialnon-transmissive to the laser beams L and may have cavities 142 and 152through which the laser beams L can pass, respectively, as illustratedin FIG. 6a . Each of the cavities 142 and 152 is in contact with thelaser beam-transmitting LED cell 130 of the micro-LED 100 at one sidethereof and is open toward the corresponding solder 263 arranged betweenthe submount substrate 200 and the micro-LED 100. When the laser beamirradiation units 1000 are operated, the laser beams L sequentially passthrough the focusing lenses 1400, the micro-LED 100, and the cavities142 and 152 of the electrode pads 140 and 150 and reach and are focusedon the solders 263. The solders 263 are hardened to connect theelectrode pads of the micro-LED 100 to the electrodes (or the pillarsformed on the electrodes) of the submount substrate 200, after beingmelted by the laser beams. The molten solders 263 fill the cavities 142and 152, enabling more reliable bonding between the electrode pads andthe electrodes.

Other Examples

As explained in the foregoing example, the plurality of laser beamirradiation units 1000 are matched to the plurality of solders in a 1:1ratio such that a plurality of laser beams L irradiated from theplurality of laser beam irradiation units 1000 heat the solders in a 1:1ratio. Other examples of the present invention are illustrated in FIG.7. Referring to FIG. 7, each of the laser beam irradiation units 1000can participate in heating several solders while moving in a certaindirection. That is, each of the laser beam irradiation units 1000 mayparticipate in heating two or more solders in a 1:n (where n is anatural number equal to or greater than 2) ratio.

FIG. 8 illustrates examples of using the laser beam irradiation unitswhose number is smaller than that of the solders. For example, each ofthe laser beam irradiation units may heat several solders 263 in alinear ((a) of FIG. 8) or zigzag pattern ((b) of FIG. 8). Alternatively,several laser beams L1, L2, L3, and L4 may heat the solders 263 arrangedin groups G1, G2, G3, and G4, as illustrated in (c) and (d) of FIG. 8.The laser beam irradiation units may heat the solder groups while movingin various patterns other than the linear ((c) of FIG. 8) or zigzagpattern ((d) of FIG. 8).

As described before, it should be understood that flip-bonding can beperformed by matching the plurality of the laser beam irradiation unitsto the plurality of the solders in a 1:1 ratio such that laser beamsirradiated from each of the plurality of laser beam irradiation unitsheat the corresponding solder, by matching the laser beam irradiationunits to the plurality of solders in a 1:n (where n is a natural numberequal to or greater than 2) ratio such that laser beams irradiated fromeach of the laser beam irradiation units heat two or more solders whilethe laser beam irradiation unit moves in a linear or zigzag pattern, orby matching two or more laser beam irradiation units to two or moregroups of the solders such that each of the laser beam irradiation unitsheats the solders in the corresponding solder group.

Particularly, the flip-bonding based on the heating of the soldersarranged in groups can provide a solution to the problems of loweconomic efficiency and inefficient work space utilization encounteredwhen laser beams are matched to solders in a 1:1 ratio and the problemof long heating and cooling time encountered when one laser beamirradiation unit is used to heat all solders.

Although the foregoing examples have been explained based on the heatingof the solders by laser beams passing through the micro-LED includingthe sapphire substrate, it is noted that soldering can be performed byirradiating laser beams onto the solders through the laserbeam-transmitting submount substrate.

Second Embodiment

Construction of Micro-LED

A micro-LED is constructed by substantially the same process asexplained with reference to FIGS. 1a to 1 e.

Preparation of Submount Substrate and Formation of Bumps

A submount substrate is prepared and bumps are formed by substantiallythe same processes as explained with reference to FIGS. 3 and 4.

Flip-Bonding

As illustrated in (a) and (b) of FIG. 9, the micro-LED 100 based on asapphire substrate 131 is flip-bonded to the submount substrate 200based on a Si substrate material. The Si substrate material has acoefficient of thermal expansion of 2.6 μmm⁻¹ K and the sapphiresubstrate 131 has a coefficient of thermal expansion of 7.6 μmm⁻¹K,which is about 2.5-fold higher than that of the Si substrate material.

As mentioned earlier, the submount substrate 200 includes a plurality ofelectrodes disposed corresponding to electrode pads 150 of the micro-LED100. The bumps 260 are formed on the plurality of electrodes. Each ofthe bumps 260 consists of a Cu pillar 262 and a SnAg solder (i.e. soldercap 263).

The micro-LED 100 is flip-bonded to the submount substrate 200 throughthe bumps so that the electrode pads 150 of the micro-LED 100 areconnected to the electrodes of the submount substrate 200.

For flip-bonding of the micro-LED 100 to the submount substrate 200, asolder constituting at least a portion of each solder bump 260 should beheated to a temperature around its melting point. However, in the casewhere a conventional flip-bonding process is performed withoutcontrolling the temperatures of the micro-LED 100 and the submountsubstrate 200, a difference in strain between the Si-based submountsubstrate 200 and the sapphire substrate 131 is observed because thecoefficient of thermal expansion of the Si-based submount substrate 200is significantly different from that of the sapphire substrate 131 ofthe micro-LED 100, leading to a severe misalignment between the submountsubstrate 200 and the micro-LED 100 flip-bonded thereto. The coefficientof thermal expansion of the sapphire substrate amounts to about 2.5-foldhigher than that of the Si-based submount substrate. The presentinvention is preferably used for flip-bonding between two substratesthat are different by a factor of 2 in coefficient of thermal expansion.

For instance, when it is desired to flip-bond the micro-LED 100 based onthe 1-cm long sapphire substrate 131 to the 1-cm long Si-based submountsubstrate 200 at 250° C. at which the solders are melted, the submountsubstrate 200 is lengthened by 5.85 μm due to the coefficient of thermalexpansion of Si and the light-transmitting sapphire substrate 131 of themicro-LED 100 is lengthened by 17.1 μm due to the coefficient of thermalexpansion of sapphire. That is, the difference in length between the twosubstrates is 11.25 μm during bonding. As a consequence, this lengthdifference causes a severe cell misalignment.

In an attempt to solve the problem of cell misalignment, the presentinvention takes into consideration the coefficient of thermal expansionof the Si-based submount substrate 200 including a drive IC and acircuit and the coefficient of thermal expansion of the sapphiresubstrate 131. Based on this consideration, the micro-LED 100 isflip-bonded to the submount substrate 200 by heating the solders 263between the micro-LED 100 and the submount substrate 200, morespecifically, the solder 263 of the bumps 260 interposed between theelectrode pads 150 formed on the LED cells 130 of the micro-LED 100 andthe submount substrate 200 while controlling the temperatures of theSi-based sapphire substrate 200 and the sapphire substrate 131 todifferent values.

The temperature of the sapphire substrate 131 is controlled by a firsttemperature control unit 5 b mounted on a first chuck 5 a inface-to-face contact with the light-transmitting sapphire substrate 131to support the micro-LED 100 and the temperature of the Si-basedsubmount substrate 200 is controlled by a second temperature controlunit 6 b mounted on a second chuck 6 a to support the submount substrate200.

The temperatures of the submount substrate 200 and the sapphiresubstrate 131 of the micro-LED 100 are controlled to different values ina heating zone A1, a holding zone A2, and a cooling zone A3 duringflip-bonding, as best illustrated in FIG. 10.

In the heating zone A1, the temperature of the light-transmittingsapphire substrate 131 is allowed to rise linearly along a first heatingcurve from room temperature to a first holding temperature (˜170-180°C.) by the first temperature control unit 5 b mounted on the first chuck5 a and the temperature of the Si-based submount substrate 200 isallowed to rise linearly along a second heating curve from roomtemperature to a second holding temperature (˜350-400° C.) by the secondtemperature control unit 6 b mounted on the second chuck 6 a.

In the holding zone A2, a vertical force is applied to pressurize thesubmount substrate 200 and the micro-LED 100 between which the moltensolders 264 are interposed, the sapphire substrate 131 is maintained atthe first holding temperature (125° C.) for the indicated time, and theSi-based submount substrate 200 is maintained at the second holdingtemperature (260° C.) for the indicated time.

In the holding zone, the sapphire substrate 131 and the submountsubstrate 200 begin to maintain their temperatures at the same point a1and finish to maintain their temperatures at the same point a2.

In the cooling zone A3, the light-transmitting sapphire substrate 131 iscooled from the first holding temperature to room temperature and theSi-based submount substrate 200 is cooled from the second holdingtemperature to room temperature. In the cooling zone A3, it is preferredthat the cooling curve of the sapphire substrate 131 is the same as thatof the Si-based submount substrate 200. Thus, in the cooling zone, thetime when the cooling of the light-transmitting sapphire substrate 131to room temperature is completed is earlier than the time when thecooling of the submount substrate 200 to room temperature is completed.

If the cooling curve of the light-transmitting sapphire substrate 131 ismade excessively different from that of the submount substrate 200 inorder to complete the cooling of the sapphire substrate 131 and thesubmount substrate 200 at the same time point, a significant differencein shrinkage strain between the sapphire substrate 131 and the submountsubstrate 200 is observed, and as a result, the solder connections arecut and the LED cells are misaligned.

What is claimed is:
 1. A method for flip-bonding a micro-LED to asubmount substrate, comprising: forming a plurality of LED cells on anLED substrate to prepare the micro-LED; preparing the submount substratehaving a coefficient of thermal expansion different from that of themicro-LED; and flip-bonding the micro-LED to the submount substratethrough solders located therebetween, wherein the submount substrate andthe micro-LED are controlled to different temperatures corresponding todifferent heating-cooling curves during the flip-bonding such that adifference in strain caused by the different coefficients of thermalexpansion of the LED substrate and the submount substrate is suppressed.2. The method according to claim 1, wherein the submount substrate andthe micro-LED are controlled to different temperatures in a heatingzone, a holding zone, and a cooling zone during the flip-bonding.
 3. Themethod according to claim 2, wherein, in the heating zone, the LEDsubstrate is heated from room temperature to a first holding temperaturealong a first heating slope and the submount substrate is heated fromroom temperature to a second holding temperature higher than the firstholding temperature along a second heating slope steeper than the firstheating slope.
 4. The method according to claim 2, wherein, in theholding zone, the LED substrate is maintained at the first holdingtemperature for an indicated time and the submount substrate ismaintained at the second holding temperature for an indicated time. 5.The method according to claim 2, wherein, in the cooling zone, the LEDsubstrate is cooled from the first holding temperature to roomtemperature and the submount substrate is cooled from the second holdingtemperature to room temperature.
 6. The method according to claim 2,wherein, in the cooling zone, the time when the cooling of the LEDsubstrate is completed is different from the time when the cooling ofthe submount substrate is completed.
 7. The method according to claim 2,wherein, in the cooling zone, the cooling slope of the LED substrate isthe same as that of the submount substrate.
 8. The method according toclaim 1, wherein the temperature of the LED substrate is controlled by atemperature control unit mounted on a chuck to fix the micro-LED duringthe flip-bonding.
 9. The method according to claim 1, wherein thetemperature of the submount substrate is controlled by a temperaturecontrol unit mounted on a chuck to fix the submount substrate during theflip-bonding.
 10. The method according to claim 1, wherein the LEDsubstrate is a sapphire substrate and each of the LED cells is formed byetching an epitaxial layer comprising an n-type semiconductor layer, anactive layer, and a p-type semiconductor layer grown on the sapphiresubstrate.
 11. The method according to claim 1, wherein the submountsubstrate comprises a Si-based substrate material, a plurality of CMOScells formed on the Si-based substrate material so as to correspond tothe plurality of LED cells, and a plurality of electrodes connected tothe plurality of CMOS cells.
 12. The method according to claim 1,wherein each of the LED cells comprises an n-type semiconductor layer,an active layer, and a p-type semiconductor layer and the micro-LED isprepared by arranging the plurality of LED cells in a matrix on anexposed area of the n-type semiconductor layer on the LED substrate. 13.The method according to claim 12, wherein the micro-LED is prepared bydisposing an n-type electrode pad on the exposed area of the n-typesemiconductor layer and disposing a p-type electrode pad on each of thep-type semiconductor layers of the LED cells.
 14. The method accordingto claim 13, wherein the flip-bonding comprises connecting the pluralityof p-type electrode pads and the n-type electrode pad to a plurality ofindividual electrodes and a common electrode disposed on the submountsubstrate through a plurality of solders, respectively.
 15. The methodaccording to claim 13, wherein the preparation of the micro-LEDcomprises forming a passivation layer so as to cover the plurality ofLED cells and the exposed area of the n-type semiconductor layer andforming holes through which the plurality of p-type electrode pads areexposed and a hole through which the n-type electrode pad is exposed.